Aiming at a line four cylinder engine，the engine mounting system is optimized by using traditional energy-decoupling method. Taking the minimization of acceleration response as objective，the engine mounting system is optimized by using the multi-body dynamic technology. The dynamic responses at supporting positions are used as evaluation index to compare the optimization results. The results indicates that there are several layout projects to reach high decoupling ratio，however，the projects do not always satisfy the requirement of vibration isolation. A principle for the design of mounting system is proposed：high decoupling ratio is not the objective of vibration isolation，while the minimization of responses under different excitations is the essential criterion evaluating the vibration isolation performance.